Generic ReadOnlyDictionary implemented in C#, for .NET < Version 4.5

ReadOnlyDictionary.cs

class ReadOnlyDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    internal class KeyCollection : ICollection<TKey> 
    {
        public KeyCollection(ICollection<TKey> collection)
        {
            if (collection == null) { throw new ArgumentNullException("collection"); }

            _collection = collection;
        }

        public IEnumerator<TKey> GetEnumerator()
        {
            return _collection.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public void Add(TKey item)
        {
            throw new NotSupportedException("Collection is read-only.");
        }

        public void Clear()
        {
            throw new NotSupportedException("Collection is read-only.");
        }

        public bool Contains(TKey item)
        {
            return _collection.Contains(item);
        }

        public void CopyTo(TKey[] array, int arrayIndex)
        {
            _collection.CopyTo(array, arrayIndex);
        }

        public bool Remove(TKey item)
        {
            throw new NotSupportedException("Collection is read-only.");
        }

        public int Count
        {
            get { return _collection.Count; }
        }

        public bool IsReadOnly
        {
            get { return true; }
        }

        private readonly ICollection<TKey> _collection;
    }

    internal class ValueCollection : ICollection<TValue>
    {
        public ValueCollection(ICollection<TValue> collection)
        {
            if (collection == null) { throw new ArgumentNullException("collection"); }

            _collection = collection;
        }

        public IEnumerator<TValue> GetEnumerator()
        {
            return _collection.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public void Add(TValue item)
        {
            throw new NotSupportedException("Collection is read-only.");
        }

        public void Clear()
        {
            throw new NotSupportedException("Collection is read-only.");
        }

        public bool Contains(TValue item)
        {
            return _collection.Contains(item);
        }

        public void CopyTo(TValue[] array, int arrayIndex)
        {
            _collection.CopyTo(array, arrayIndex);
        }

        public bool Remove(TValue item)
        {
            throw new NotSupportedException("Collection is read-only.");
        }

        public int Count
        {
            get { return _collection.Count; }
        }

        public bool IsReadOnly
        {
            get { return true; }
        }

        private readonly ICollection<TValue> _collection;
    }

    public ReadOnlyDictionary(IDictionary<TKey, TValue> dictionary)
    {
        _dictionary = dictionary;
    }

    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
    {
        return _dictionary.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public void Add(KeyValuePair<TKey, TValue> item)
    {
        throw new NotSupportedException("Collection is read-only.");
    }

    public void Clear()
    {
        throw new NotSupportedException("Collection is read-only.");
    }

    public bool Contains(KeyValuePair<TKey, TValue> item)
    {
        return _dictionary.Contains(item);
    }

    public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        _dictionary.CopyTo(array, arrayIndex);
    }

    public bool Remove(KeyValuePair<TKey, TValue> item)
    {
        throw new NotSupportedException("Collection is read-only.");
    }

    public int Count
    {
        get { return _dictionary.Count; }
    }

    public bool IsReadOnly
    {
        get { return true; }
    }
        
    public bool ContainsKey(TKey key)
    {
        return _dictionary.ContainsKey(key);
    }

    public void Add(TKey key, TValue value)
    {
        throw new NotSupportedException("Collection is read-only.");
    }

    public bool Remove(TKey key)
    {
        throw new NotSupportedException("Collection is read-only.");
    }

    public bool TryGetValue(TKey key, out TValue value)
    {
        return _dictionary.TryGetValue(key, out value);
    }

    public TValue this[TKey key]
    {
        get { return _dictionary[key]; }
        set { throw new NotSupportedException("Collection is read-only."); }
    }

    public ICollection<TKey> Keys
    {
        get { return _keys ?? (_keys = new KeyCollection(_dictionary.Keys)); }
    }

    public ICollection<TValue> Values
    {
        get { return _values ?? (_values = new ValueCollection(_dictionary.Values)); }
    }

    private KeyCollection _keys;
    private ValueCollection _values;
    private readonly IDictionary<TKey, TValue> _dictionary;
}